The so-called horizontal axis wind turbines, used for transforming wind energy into electricity, comprise, as is already known, a tower, a nacelle and a wind rotor having at least two blades. Due to the action of the wind incident upon the blades, the rotor rotates around an axis which orientation is substantially horizontal with respect to the ground. In turn, the nacelle can rotate around the longitudinal axis of the tower to orient the rotor axis windward (in upwind-type wind turbines, where the wind firstly strikes the rotor and then the tower that supports the gondola) or leeward (in downwind-type wind turbines where the wind firstly strikes the tower and then the rotor).
Moreover, offshore wind energy is currently under development. In this case, the wind turbines are installed at sea (or on lakes), either on support structures fixed to the seabed or supported by floating platforms.
There are trends, in current wind turbine design, aimed at increasing blade length to increase the rated power of the wind turbines and to increase annual energy production per wind turbine, as well as at manufacturing the blades from more flexible and lighter materials for the purpose of obtaining a convenient weight/rigidity ratio.
This last design trend has the drawback that, in upwind-type horizontal-axis wind turbines, deflection of the blades produced by strong winds tends to push the tip of the blades towards the tower, which would cause irreparable damage to the blades and to the tower in case of contact therewith.
In order to solve this problem, one solution would be to provide the blades with sufficient rigidity to minimise their deflection. However, providing the blades with a high degree of rigidity implies increasing their weight and therefore price. Additionally, the greater weight of the rotor implies an increase in the cost of other structural parts of the wind turbine and platform.
Another possible solution consists of disposing the axis forming a small angle with respect to the horizontal, called tilt angle, in such a manner that during rotation the tip of the blades is farther away from the tower on passing in front of said tower.
A double drawback of using rotors mounted with a tilt angle greater than zero is that the performance of the machine is modified proportionally to the cube of the cosine of the tilt angle, due to the fact that the wind acquires a component not parallel to the rotor axis, in addition to the fact that said non-parallel component produces additional loads on the wind turbine.
Table 1 below illustrates the relationship between the tilt angle, the cube of the cosine of said tilt angle and the loss of power resulting from having said angle with respect to an angle equal to zero, at a wind speed where the captured power is less than the rated power of the wind turbine, obtained from the equation for power captured by the wind turbine given by the approximate formula:
  P  =            1      2        ·    ρ    ·          A      1        ·    Cp    ·          v      3        ·                  cos        3            ⁡              (        tilt        )            where P is the captured power, ρ is air density, A1 is the surface area swept by the blades, Cp is a coefficient associated with the blades and v is wind speed.
TABLE 1Relationship between loss of captured power and tilt angle.TiltCos3(tilt)Loss20.9981735940.18%40.9927099380.73%60.0936555511.63%80.9710874162.89%100.9551121664.49%120.9358649496.41%140.9135080018.65%
On the other hand, as can be observed in FIG. 1, the wind that strikes the blades of a wind turbine disposed on a floating platform exerts a driving force which has the effect of leaning the tower leeward at a given tower pitch angle which, in the case of upwind generators, increases the rotor axis orientation angle, moving from being equal to the tilt angle to being equal to the sum of said tilt angle plus the tower pitch angle, with the ensuing additional loss of efficiency, as previously explained.
The technical problem addressed consists of describing a device and method for controlling the orientation of a floating platform for an upwind-type wind turbine having a rotor axis with a tilt angle greater than zero, allowing the maximising of the wind turbine efficiency.